Chlorine ranks among the ten most important commodity chemicals produced worldwide. The total production of chlorine in the United States in 1991 was reported to be about 14 million tons, almost all of which was produced by the electrolysis of brine. The product of electrolysis is chlorine gas, contaminated with water, hydrogen, air, and other impurities. After the removal of water and other impurities, most chlorine is liquefied by compression and chilling, then sold. As with all compression/condensation processes, it is difficult to recover all the condensable chlorine gas without going to extreme conditions of temperature and pressure. It is not unusual, therefore, for the tail gas from the liquefaction process to contain as much as 40% chlorine.
The presence of hydrogen in the gas stream is an added complication. When hydrogen is present in a gas stream with chlorine or with oxygen at hydrogen concentration less than about 4%, dependent upon pressure and temperature, usually the stream is non-explosive. However, as the hydrogen concentration increases above this lower explosive limit, the reaction on ignition becomes more violent and eventually may reach the detonation stage. To avoid this, the gas stream is routinely diluted with enough air or nitrogen to keep the hydrogen concentration below the 4% limit. Typically such additions are made after condensation steps, wherein the condensable components are removed, leaving a higher concentration of hydrogen in the vent stream.
For the past forty years, the tail gas has been treated by absorption in carbon tetrachloride. Tail gas from chlorine liquefaction, and other waste streams ("sniff gas") from the plant, are supplied to the carbon tetrachloride absorber under pressure. Chlorine-free (.about.1 ppm) gas is vented to the atmosphere. The chlorine-rich carbon tetrachloride is fed to a stripper, where chlorine is desorbed and sent to the liquefaction system. The stripped solvent is pumped back to the absorption tower.
Approximately 30 lb of carbon tetrachloride per ton of recovered chlorine are lost in this process. It is estimated that 9 million lb of carbon tetrachloride are emitted annually by chlorine liquefaction tail-gas treatment plants. Additional emissions remit from similar chlorine absorption processes used in the paper, textile, and polyvinyl chloride industries. Because of the high ozone-depletion potential of carbon tetrachloride, the U.S. Environmental Protection Agency has mandated that these emissions be eliminated, and carbon tetrachloride production will cease after 1995. There is an urgent need, therefore, for alternative treatment technology.
A number of metals are also produced by the electrolysis of their molten chlorides, for example, magnesium, calcium, beryllium, and sodium. In all cases, chlorine-containing gas is liberated at the cell anodes; in magnesium production, the gas may contain as much as 90% chlorine. Other processes that require removal or recovery of chlorine from gas streams include, but are not limited to: production of chlorinated chemicals, bleaching, refrigeration and heat transfer fluids, chlorine transfer and clean-up operations, ore beneficiation, and wastewater treatment.
Gas separation by means of membranes is known. For example, U.S. Pat. No. 4,230,463, to Henis and Tripodi, describes multicomponent membranes for separating oxygen from air. U.S. Pat. Nos. 4,180,552, to Graham and MacLean, 4,180,553, to Null and Perry, and 4,654,063, to Auvil and Agrawal, describe membrane processes for separating hydrogen from various gas streams. Separation of carbon dioxide from natural gas is taught in U.S. Pat No. 4,130,403, to Cooley and Coady. U.S. Pat. No. 4,553,983, to Baker, describes methods for removing organic vapor from air, using highly organic-selective membranes.
Many of these membranes and membrane processes have been in use for 10-15 years in various commercial applications. However, to date, membranes have not been used for the separation of chlorine from other gases, probably because of the known or expected extreme reactivity of chlorine gas with many of the polymers typically used in making membranes. In fact, the inventors are not aware of any permeability data on chlorine separation. Until now, it was not known whether a membrane could be made that would be selective for chlorine over other gases. In addition, chlorine is highly corrosive to many of the materials often used in building membrane systems--aluminum, polyvinyl chloride (PVC), silicones, epoxies, and so on. With the exception of steel, which is the commonly-used material for containers for storage or transport of liquid chlorine, many materials that are in everyday, general use in membrane separation systems have very limited resistance to chlorine gas or liquid.
The difficulties encountered when operating polymer membranes in the presence of chlorine are exemplified by the reverse osmosis (RO) industry. Various membranes made from cellulose acetates, polyamides, and polyetherureas, among others, are used for commercial RO applications; the industry has experimented with many more. Most membranes, commercial or developmental, have moderate-to-limited-to-nil resistance to the chlorine used as a pre-treatment to kill microorganisms that would otherwise foul the membrane surface. After chlorine treatment, the feedstream must be dechlorinated prior to membrane contact, a costly addition to the overall treatment process. Thus, since the beginnings of the RO industry, the membrane community has been involved in an on-going search for chlorine-resistant membranes for use in RO processes. A few representative patents relating to this problem are U.S. Pat. Nos. 4,302,336; 4,661,254; 4,913,816; 4,941,972 and 5,013,448.
Applicants are aware of only one publication that concerns gaseous chlorine permeation through polymeric films. This is a paper in the September, 1979, issue of Environmental Science and Technology, entitled, "A Personal Chlorine Monitor Utilizing Permeation Sampling," by Hardy et at. This paper describes a method of determining personal exposure to low levels of chlorine by equipping individuals with a small device the size of a radiation dosimeter, to be worn on the belt, for example. The device contains 10 ml of fluorescein-bromide absorbing solution, which is converted to eosin in the presence of chlorine. The amount of eosin present after a certain period of exposure can be measured and used to calculate the amount of chlorine to which the wearer has been exposed. A 25-.mu.m silicone rubber membrane enables chlorine to enter the device, while preventing the absorbing solution from falling out.
Permeation of chlorine through the membrane into the device is concentration driven; no external driving force is provided. Thus permeation can be slow; as the reference points out, "the response may take several hours." The magnitude of the transmembrane flux does not matter, so long as some chlorine can pass into the device and the resulting eosin content can be compared with a previously made calibration curve. Furthermore, the membrane is not there to separate or concentrate chlorine from other components of the air--it simply provides a pathway by which chlorine can enter the device and come into contact with the liquid contained therein. The reference is silent as to any separating properties that the membrane may or may not have with regard to chlorine.
Also, since the device is only exposed to extremely low chlorine concentrations and would only be used for a few hours, stability of the membrane in the presence of chlorine is not an issue and is not discussed.
Thus, the known aggressive chemical properties of chlorine and the known membrane degradation problems experienced in RO applications would suggest that chlorine separation from gas or vapor streams is not a good candidate for a membrane-based system. To applicants' knowledge, there is an absence of any gaseous permeation or separation data available in the art.